Artist’s concept of TRAPPIST-1 planets, emphasizing the dimensions of their diameters and masses. The TRAPPIST-1 galaxy is home to the largest batch of approximately terrestrial planets ever found outside of our solar system. It is about 40 light years away. Image via NASA / JPL-Caltech / University of Geneva.
An exciting discovery by the Spitzer Space Telescope in 2017 revealed seven Earth-sized planets orbiting the nearby star TRAPPIST-1, less than 40 light-years away. Details about these planets are hard to come by, but astronomers wondered: Are there any like Earth? Are some like Venus? Are there clouds or even surface water? Late last week (January 22, 2021), astronomers added one more piece to the puzzle of the Trappist-1 planets when they published an article in the peer-reviewed Planetary Science Journal detailing the compositions of these worlds. The team found that they were made of the same material, but very different from the planets in our solar system.
Trappist-1 is a red dwarf star, by far the most common type of star in our Milky Way galaxy. Three of the Trappist-1 planets reside firmly within the star’s habitable zone – also known as the Goldilocks Zone – in which liquid water can exist on a planet’s surface. At this point, astronomers feel that water is unlikely to be found on the surfaces of Trappist-1’s three inner planets, and if the four outer planets do have surface water, that’s not much. The discovery contradicts an early belief among astronomers that low-density planets like this one must be abundant in water. It also raises questions about how earthy and habitable the seven exoplanets could be.
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Astrophysicist Martin Turbet of the University of Geneva is a co-author of the new study. He said in a statement:
By combining the planetary interior models of the Universities of Bern and Zurich with the planetary atmosphere models that we are developing at the University of Geneva, we were able to evaluate the water content of the seven TRAPPIST-1 planets with a precision that is literally unprecedented. for this category. of the planet.
Our internal and atmospheric structural models show that the three inner planets of the TRAPPIST-1 system are probably waterless and that the four outer planets have no more than a few percent water, possibly in liquid form, on their surfaces.
Now let’s think about the eight planets in our solar system. They vary widely in density, with the large gas and ice giants (Jupiter, Saturn, Uranus, Neptune) being less dense and the terrestrial planets (Mercury, Venus, Earth, Mars) being denser. Because the seven bodies in TRAPPIST-1 were known to have a relatively similar density to each other, researchers ran into a dilemma when comparing these exoplanets to Earth. If one of them did indeed have surface water, astronomers’ estimates of density would have to change.
Therefore, at the start of the study, researchers examined whether the lower density of the seven exoplanets was the result of water or their inner composition, both of which would determine whether the planets are very or only slightly terrestrial in nature. . According to a statement on the study, the exoplanets are about 8% less dense than Earth. For the lower density of the planets to be the result of surface water, about 5% of each planet’s mass should exist as surface water.
That’s a lot! In comparison, water only represents 0.1 percent of the earth’s mass. So you can immediately see that these worlds are quite different from the earth in terms of composition.
Martin Turbet is an astronomer at the University of Geneva and a co-author of the study that calculated the densities of the exoplanets in TRAPPIST-1. Image via University of Geneva.
Eric Agol is an astronomer at the University of Washington and the lead author of the study that calculated the densities of the exoplanets in TRAPPIST-1. Image via The Daily at the University of Washington.
But it turns out that even the potential for a few percent of the body of water is questionable. Astronomer Eric Agol of the University of Washington is the lead author of the study. He added in the statement that the planets probably have less than a few percent water mass; otherwise the comparable densities within the group would be an extraordinary coincidence.
To answer this dilemma, researchers examined the composition of the seven exoplanets, looking for similarities within the group and between the group and Earth. Most rocky planets are believed to be made of metals such as magnesium, iron and nickel, and non-metals such as sulfur, oxygen, and silicon. Due to the discrepancy in density between the group and Earth, researchers suspected that the TRAPPIST-1 exoplanets have a similar composition to Earth, but with significantly different ratios, for example, a lower percentage of iron. Although iron represents 32% of the Earth’s total mass, the study indicates that iron should represent about 21% of the mass of each TRAPPIST-1 exoplanet.
Agol told EarthSky:
The lower density indicates that these planets have a different composition, and thus a different history, than the terrestrial planets of our solar system. Maybe the star + disk started with less iron? Or maybe the iron core never formed and the iron remained oxidized in the mantle?
Artist’s concept of the comparable densities of the 7 exoplanets in the TRAPPIST-1 star system. Their similar density suggests that they all have a similar composition. The new study has led scientists to believe the planets have rocky surfaces and iron-rich cores. Their cores are likely smaller than Earth’s, as the planets each have a mass about 8% less than Earth’s. Previous theories involved deep ocean layers on the surface of each planet, or planets with no nucleus. Image via NASA / JPL-Caltech / University of Geneva.
To determine the mass of each planet, scientists measured changes in their orbital time. They measured how long it took for each planet to orbit TRAPPIST-1 and before the star had crossed over from Earth. This process is called transit timing. When combined with radius measurements of each planet, scientists were able to more accurately calculate the densities of the planets. Using an equation with Earth’s mass, the team was able to calculate the percentage of iron likely to be present in each of the seven exoplanets.
Using data from the now-decommissioned Spitzer Space Telescope, astronomers have determined that all 7 planets orbiting TRAPPIST-1 have a similar density. This finding helped the astronomers characterize the composition of these exoworlds in comparison to Earth. While all seven planets are considered to be roughly the size of Earth, they are each about 8% less dense than our home planet, meaning that, while potentially similar to Earth, in terms of elements such as iron, their composition differs significantly in percentages . Image via NASA / JPL-Caltech / University of Geneva.
By combining this data with radius measurements of each planet, scientists calculated the planets’ densities with more precision than before. Using an equation with Earth’s mass, the team was able to calculate the percentage of iron likely to be present in each of the seven exoplanets.
Agol said:
We found that about 2/3 of the iron is needed compared to Earth, because they have a lower density.
Whether the differences between Earth and the TRAPPIST planets change the potential for life somewhere in the TRAPPIST-1 system is a complicated question. While the presence of liquid water could indicate the potential of life, other factors contribute to a planet’s fitness for life to emerge, thrive, and survive. On Earth, for example, a strong magnetic field protects our planet and life from high-energy particles from the sun. Our atmosphere is filled with enough oxygen and carbon, as well as other gases necessary for animal life, photosynthesis, and habitable surface temperatures.
Despite these challenges, Agol has not yet ignored the potential of life on TRAPPIST-1 exoplanets. He said:
The link with habitability is not yet clear. The structure of the planets can affect their ability to have plate tectonics, carry a magnetic field, and other possible implications. These aspects of the Earth influence the presence and change of life on Earth.
So there are still many questions to investigate.
The study was made possible by data sets created with the Spitzer Space Telescope since the galaxy was discovered more than four years ago. Between 2016 and 2020, when the telescope was decommissioned and stopped collecting data, Spitzer recorded 1,075 hours of observation time for TRAPPIST-1. While Spitzer’s decommissioning has sustained the observations, it doesn’t mean a definitive end to TRAPPIST-1 studies.
Agol said:
If the James Webb space telescope is successfully launched and commissioned later this year, we plan to continue to monitor the transits with that telescope to try to detect the atmospheres with transmission spectroscopy. Each transit provides timing so that we can continue to refine the masses of the planets.
After the discovery of exoplanets orbiting TRAPPIST-1 in 2016, the Spitzer Space Telescope recorded 1,075 hours of observation on the galaxy, allowing astronomers to collect enough data to accurately determine the density and composition of each planet. The Spitzer space telescope was decommissioned in January 2020. Artist’s concept via NASA / JPL-Caltech.
In short, a new study of the seven Earth-sized exoplanets in the TRAPPIST-1 system indicates that all seven planets are very similar in composition, but may be quite different from Earth.
Source: Refinement of the transit timing and photometric analysis of TRAPPIST-1: masses, rays, densities, dynamics and ephemeris
Via Unibe.ch
